It modifies the output volume progressively, thus the volume is high from the moment of triggering, then it is attenuated until it becomes null and there is no sound.
The setting of the decay time of the volume is done via a potentiometer. If it is at the minimum, it will produce a very short sound, else it will produce a very long sound after triggering.
The triggering is done by hitting the piezzo sensor (most of the time through the box in wich it is setup).
The sensivity of triggering is also settable (with a screwdriver) by a trimmer potentiomer on the board.